A magnetic bearing is used in a rotary machine, such as a turbo-molecular pump, that is used in a semiconductor manufacturing step. A conventional magnetic bearing excitation circuit is now described based on a configuration example of a magnetic bearing of a turbo-molecular pump.
FIG. 4 shows a cross-sectional view of the turbo-molecular pump, a configuration example of a magnetic bearing. In FIG. 4, the turbo-molecular pump has a rotor 103 having a plurality of stages of rotor blades 101a, 101b, 101c and the like, which are turbine blades for discharging gas.
A magnetic bearing is configured by disposing an upper radial electromagnet 105a, a lower radial electromagnet 107a, and axial electromagnets 109a, to bear the rotor 103. The turbo-molecular pump also has an upper radial sensor 105b, a lower radial sensor 107b, and an axial sensor 109b. 
The upper radial electromagnet 105a and lower radial electromagnet 107a each have four electromagnets constituted by electromagnetic windings, as shown in a lateral cross-sectional view of FIG. 5. These four electromagnets are disposed in pairs facing one another, to configure a magnetic bearing along two X and Y axes.
Specifically, electromagnetic windings 111 wound around two adjacent core projecting parts are disposed in a pair, with the polarities thereof reversed, thereby configuring a single electromagnet. This electromagnet forms a pair with an electromagnet that is constituted by electromagnetic windings 113 of core projecting parts facing the electromagnetic windings 111 with the rotor 103 therebetween, and these electromagnets pull the rotor 103 in a positive or negative direction along the X axis.
Similarly, in the Y axis perpendicular to the X axis, two electromagnetic windings 115 and two electromagnetic windings 117 facing the electromagnetic windings 115 form a pair of electromagnets facing each other in the direction of the Y axis.
As shown in a longitudinal cross-sectional view of FIG. 6, the axial electromagnets 109a are configured as a pair of electromagnets, with two electromagnetic windings 121, 123 thereof having an armature 103a of the rotor 103 therebetween. These two electromagnets 109a configured respectively with the electromagnetic windings 121, 123 apply suction force in a positive or negative direction of the axis of rotation, to the armature 103a. 
The upper radial sensor 105b and the lower radial sensor 107b each consist of four sensing coils corresponding to the electromagnets 105a, 107a and disposed along the two X and Y axes, and detect a radial displacement of the rotor 103. The axial sensor 109b detects an axial displacement of the rotor 103. These sensors are configured to send the detection signals thereof to a magnetic bearing control device, not shown.
Based on the detection signals of these sensors, the magnetic bearing control device executes PID control and the like to individually adjust the suction forces of a total of ten electromagnets configuring the upper radial electromagnet 105a, the lower radial electromagnet 107a, and the axial electromagnets 109a, thereby magnetically suspending and supporting the rotor 103.
Next is described a magnetic bearing excitation circuit that excites and drives each of the electromagnets of the magnetic bearing as described above. FIG. 7 shows an example of a magnetic bearing excitation circuit that controls a current flowing through the electromagnetic windings by means of a pulse width modulation system.
As shown in FIG. 7, one of the electromagnetic winding 111 configuring a single electromagnet has one end thereof connected to a positive electrode of a power supply 133 by a transistor 131 and has the other end connected to a negative electrode of the power supply 133 by a transistor 132.
A cathode of a current regeneration diode 135 is connected to the former end of the electromagnetic winding 111, and an anode of the same is connected to the negative electrode of the power supply b 133. Similarly, a cathode of a diode 136 is connected to the positive electrode of the power supply 133, and an anode of the same is connected to the latter end of the electromagnetic winding 111. An electrolytic capacitor 141 is connected for stabilization between the positive and negative electrodes of the power supply 133.
A current detection circuit 139 is provided at a source side of the transistor 132, and a current detected by this current detection circuit 139 is input to a control circuit 137.
An excitation circuit 110 with the foregoing configuration corresponds to the electromagnetic windings 111. The same excitation circuit 110 is configured for the other electromagnetic windings 113, 115, 117, 121, 123. Therefore, in a 5-axis control magnetic bearing, a total of ten excitation circuits 110 are connected in parallel with the electrolytic capacitor 141.
In this configuration, turning both of the transistors 131, 132 ON leads to an increase of the current, while turning both of the transistors OFF leads to a decrease of the current. On the other hand, a flywheel current is held by turning either one of the transistors ON. Application of the flywheel current can reduce hysteresis loss and keep power consumption low.
Moreover, high-frequency noise such as higher harmonics can be reduced. An electromagnetic current IL flowing through the electromagnetic winding 111 can be detected by measuring this flywheel current with the current detection circuit 139. The control circuit 137 determines a pulse width per cycle through pulse width modulation by comparing a current command value with a detected value obtained by the current detection circuit 139, and transmits a resultant signal to gates of the transistors 131, 132.
In a case where the current command value is greater than the detected value, both of the transistors 131, 132 are turned ON once for a period of time equivalent to a pulse width time period Tp1 within one cycle Ts (e.g., Ts=100 μs), as shown in FIG. 8. At this moment, the electromagnetic current IL increases.
However, when the current command value is lower than the detected value, both of the transistors 131, 132 are turned OFF once for a period of time equivalent to a pulse width time period Tp2 within the cycle Ts, as shown in FIG. 9. At this moment, the electromagnetic current IL decreases.
Here, a pulse width Tp is obtained using the current command value IR, the electromagnetic current IL, an electromagnetic inductance Lm, an electromagnetic resistance Rm, and a power supply voltage Vd. According to Kirchhoff's laws, Equation 1 is established between the electromagnetic current IL flowing through the electromagnetic winding 111 and the power supply voltage Vd.
                              Lm          ×                      dIL            dt                          =                  Vd          -                      IL            ×            Rm                                              [                  Equation          ⁢                                          ⁢          1                ]            
Therefore, the pulse width Tp necessary to change the current value by IR-IL can be obtained by Equation 2.
                    Tp        =                              Lm            ×                          (                              IR                -                IL                            )                                            Vd            -                          IL              ×              Rm                                                          [                  Equation          ⁢                                          ⁢          2                ]            
The power supply voltage Vd here is reduced by an AC input power supply 1 and thereafter by an AC/DC main power supply 3 and a DC/DC converter 5. This power supply voltage Vd is input to an electromagnetic power amplifier 7 and used as a power supply of the excitation circuit 110 (see Japanese Patent Application Laid-open No. 2003-293980).
Note that an output of the AC/DC main power supply 3 is input to a motor drive circuit 9 to supply power to a motor 121. An output of the DC/DC converter 5 is input to a small auxiliary power supply 11 and then formed into a control power supply of 5 V, +15 V, −15 V and the like, which is transmitted to the control circuit 137. The control circuit 137 has a built-in digital signal processor (DSP) 15.
As described above, a reduced voltage obtained through the DC/DC converter 5 is used as the power supply voltage Vd. Therefore, compared with an output voltage of the AC/DC main power supply 3 that fluctuates significantly depending on the rotational state of the motor 121, such as its acceleration or deceleration state, the power supply voltage Vd is constantly stable. Hence, in the conventional configuration, the output of the electromagnetic power amplifier 7 can be controlled stably without taking the fluctuations of the power supply voltage into much consideration. However, the conventional power supply device is equipped with such a DC/DC converter 5 for obtaining the power supply voltage Vd, and is therefore large and has a costly circuit. In addition, the conventional power supply device has a large number of parts, hence the higher failure rate.
Further, a product having a control device integrated with a vacuum pump has become mainstream recently but does not have enough circuit-mounting space. Thus, it is crucial to reduce the size of the product. In this regard, there is an example in which an electromagnetic power amplifier of a bearingless motor is driven with a high voltage of a main power supply without using a DC/DC converter (see Japanese Patent Application Laid-open No. 2010-200524). In this example, however, fluctuations of the voltage power supply are not taken into much consideration, leading to deterioration of the stability of the magnetic bearing.